.\"
.\" Copyright (c) 2009 Bruce Simpson.
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.\" $FreeBSD$
.\"
.Dd March 9, 2009
.Dt IGMP 4
.Os
.Sh NAME
.Nm igmp
.Nd Internet Group Management Protocol
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netinet/in.h
.In netinet/in_systm.h
.In netinet/ip.h
.In netinet/igmp.h
.Ft int
.Fn socket AF_INET SOCK_RAW IPPROTO_IGMP
.Sh DESCRIPTION
.Tn IGMP
is a control plane protocol used by IPv4 hosts and routers to
propagate multicast group membership information.
Normally this protocol is not used directly, except by the kernel
itself, in response to multicast membership requests by user
applications.
Routing protocols may open a raw socket to directly interact with
.Nm .
.Pp
As of
.Fx 8.0 ,
IGMP version 3 is implemented.
This adds support for Source-Specific Multicast (SSM), whereby
applications may communicate to upstream multicast routers that
they are only interested in receiving multicast streams from
particular sources.
.\"
.Sh SYSCTL VARIABLES
.Bl -tag -width indent
.\"
.It net.inet.igmp.stats
This opaque read-only variable exposes the stack-wide IGMPv3
protocol statistics to
.Xr netstat 1 .
.\"
.It net.inet.igmp.ifinfo
This opaque read-only variable exposes the per-link IGMPv3 status to
.Xr ifmcstat 8 .
.\"
.It net.inet.igmp.gsrdelay
This variable specifies the time threshold, in seconds, for processing
Group-and-Source Specific Queries (GSR).
As GSR query processing requires maintaining state on the host,
it may cause memory to be allocated, and is therefore a potential
attack point for Denial-of-Service (DoS).
If more than one GSR query is received within this threshold,
it will be dropped, to mitigate the potential for DoS.
.\"
.It net.inet.igmp.default_version
This variable controls the default version of IGMP to be used on all links.
This sysctl is normally set to 3 by default.
.\"
.It net.inet.igmp.legacysupp
If this variable is non-zero, then IGMP v1 and v2 membership reports
received on a link will be allowed to suppress the IGMP v3 state-change
reports which would otherwise be issued by this host.
This sysctl is normally enabled by default.
.\"
.It net.inet.igmp.v2enable
If this variable is non-zero, then IGMP v2 membership queries will be
processed by this host, and backwards compatibility will be enabled
until the v2 'Old Querier Present' timer expires.
This sysctl is normally enabled by default.
.\"
.It net.inet.igmp.v1enable
If this variable is non-zero, then IGMP v1 membership queries will be
processed by this host, and backwards compatibility will be enabled
until the v1 'Old Querier Present' timer expires.
This sysctl is normally enabled by default.
.\"
.It net.inet.igmp.sendlocal
If this variable is non-zero, then IGMP state-changes for groups in
the 224.0.0.0/24 link-scope prefix will be issued.
This behaviour is recommended if deploying
.Fx
in a network environment with layer 2 devices which snoop IGMP traffic
to mitigate multicast propagation throughout the network.
This sysctl is normally enabled by default.
.\"
.It net.inet.igmp.sendra
If this variable is non-zero, then IGMP v2 and v3 reports will contain
the IP Router Alert option.
This sysctl is normally enabled by default.
.\"
.It net.inet.igmp.recvifkludge
If this variable is non-zero, then received IGMP reports which contain
0.0.0.0 as their source will be rewritten to contain the subnet address.
This is useful when there are hosts on-link which have not yet been
configured with a primary IPv4 address.
This sysctl is normally enabled by default.
.\"
.El
.Sh SEE ALSO
.Xr netstat 1 ,
.Xr sourcefilter 3 ,
.Xr inet 4 ,
.Xr multicast 4 ,
.Xr ifmcstat 8
.Sh HISTORY
The
.Nm
manual page re-appeared in
.Fx 8.0 .